Thermostats having electronics, such as programmable thermostats, may rely on an independent power source, such as a disposable battery. However, a disposable battery eventually needs to be replaced by the user. Electronic thermostats can also be powered directly from an HVAC system transformer such as using a 24 VAC “common” wire (“C-wire”) from the transformer, but only if one is available. When provided, the C wire has the particular purpose of supplying power for an electronic thermostat. However, many HVAC installations do not have a C-wire provided to the thermostat. For such cases, many electronic thermostats have been designed to extract power from the transformer from the circuit used to turn on and off the HVAC function which is called “power stealing,” “power sharing” or “power harvesting.” The thermostat “steals,” “shares” or “harvests” its power during the “OFF” or “inactive” periods of the heating or cooling system by allowing a small amount of current to flow through it into the load coil below its response threshold (even at maximum transformer output voltage). During the “ON” or “active” periods of the heating or cooling system the thermostat can be designed to draw power by allowing a small voltage drop across itself. Hopefully, the voltage drop will not cause the load coil to dropout below its response threshold (even at minimum transformer output voltage). Examples of thermostats with power stealing capability include the Honeywell T8600, Honeywell T8400C, and the Emerson Model 1F97-0671. Some thermostats are designed to be operable for a variety of power sources. For example, U.S. Pat. No. 6,886,754 discusses a thermostat operable from battery, common-wire or power-stealing depending upon the type of HVAC system the thermostat is installed with.
However, a trade-off inherently exists between drawing enough power during power stealing so as to provide adequate power for thermostat operation and drawing too much power such that the power stealing causes false switching: where the HVAC function unintentionally turns on, or activates, or unintentionally turns off. Determining how much current to draw during power stealing is also complicated by the wide variety of HVAC systems with which the thermostat may be installed, as well as the desire to design a thermostat that is relatively easy to install.
Some attempts have been made to decrease the power usage of the thermostat. For example, U.S. Pat. No. 7,755,220 discusses power stealing for a thermostat using a triac with FET control. The discussed electronic thermostat circuit topology attempts to minimize the current needed to control the thermostat outputs. However, for advanced and user-friendly functions that consume more power, greater amounts of power will need to be obtained through power stealing.